Much research has been undertaken to increase the production rate of synthetic fibers and decrease the production costs by performing high-speed spinning without the necessity of any drawing step. A substantial portion of the reports published in this area is directed to polyester fibers such as polyethylene terephthalate fibers, which are easier to handle than polyamide fibers, because of such advantages as the absence of swelling problems. However, fibers having satisfactory performance are not attainable by simply increasing the spinning speed. The fiber strength increases with increasing spinning speed and reaches a maximum at a speed in the neighborhood of 6,000 m/min but as the spinning speed is increased further, the fiber strength gradually decreases. On the other hand, the fiber elongation decreases with increasing spinning speed, and no fiber is attainable that is fully satisfactory in terms of both strength and elongation. Instead of increasing the spinning speed to as high as 6,000 m/min, U.S. Pat. No. 2,604,667 describes a speed of 5,800 m/min (6,350 Y/min) in order to make a polyester fiber having a strength of from 3.2 to 4.6 g/d and an elongation of from 38 to 72%. However, this method does not employ a heat treatment during fiber making so that the fiber produced will experience a great variation in thermal shrinking stress under varying temperature conditions that are encountered in heat treatments in subsequent processing. This causes unevenness in the tension being applied to the filament yarn and increases the chance of unevenness of occurring in various aspects of the yarn such as crimp, diameter, and dye absorption.
Two methods have been proposed for producing fibers that satisfy both strength and elongation requirements; according to one proposal, the fiber being subjected to high-speed spinning is treated with steam or dry heat at a stage prior to contact with the take up roller without forcing the fiber to be drawn out between rollers as described, for example, in Japanese Patent Application (OPI) Nos. 140117/81 and 126318/85 (the term "OPI" as used herein means "unexamined published patent application"), and Japanese Patent Publication Nos. 1932/70 and 11767/80; the other method may be described as "super-high speed spinning" which simply consists of winding up the yarn at a speed not lower than 6,000 m/min as described, for example, in Japanese Patent Application (OPI) Nos. 133216/82 and 66507/84.
In the first method, the filaments are subjected to non-contact heating as they travel at high speed under low tension, so they cannot be heated uniformly, and unevenness of yarn is liable to occur. The second method is capable of reducing the fiber elongation as the spinning speed increases, but the strength of the fiber produced is inferior to that of the drawn fiber produced by the two-step spin-windup-draw process.
According to the method of the first category, described in Japanese Patent Publication No. 1932/70, a fiber having an elongation of up to 50% is produced by effecting heat treatment at a temperature of at least 80.degree. C., taking up the spun filaments at 4,000 m/min or faster, and subjecting the filaments to another heat treatment under tension. In this method, the first heat treatment is conducted after the travelling filaments have solidified upon cooling to 80.degree. C. or below, and the filaments are greatly influenced by concomitant flows because of their high travelling speed. As a result, the combined filaments will often fail to be heated uniformly. In addition, the need to effect heat treatment in two stages adds to the production cost.
Japanese Patent Publication No. 11767/80 describes a method for producing a high-strength fiber by heating spun filaments at a stage between cooling and contact with the take up roller. However, in this method a heating tube is situated immediately below the cooling section, so that unevenness of yarn will result because of the difficulty that is involved in maintaining a constant temperature of the heating tube, due to phenomena such as the carry-over of cooling air.
As an alternative to the first and second methods having the aforementioned problems, a process of "coupled spin-drawing" which involves continuous drawing of spun filaments without winding them up may be used to produce a fiber having superior characteristics in terms of not only yarn uniformity but also strength and elongation. Various proposals have been made in order to implement this process, and British Patent 1,375,151 describes a method wherein spun filaments that have been taken up at 3,000 m/min or faster are stretched at draw ratios of from 1.3 to 1.8 (i.e., 1.3/1 to 1.8/1) in a heated atmosphere of from 100.degree. to 220.degree. C. However, this method involves high-speed drawing for high draw ratios and the heating employed is indirect rather than direct, so that the temperature distribution of filaments has a tendency to become nonuniform and a fixed draw point cannot be established. Japanese Patent Application (OPI) No. 163414/84 describes a method wherein a fiber having a birefringence of 30.times.10.sup.-3 or more is subjected to continuous heat treatment and drawing. This method, however, is not economical since it requires two heating steps.
Japanese Patent Application (OPI) No. 134019/85 discloses a method wherein a fiber that has been drawn at a ratio of up to 3.0 is heat-treated and subsequently wound up at a speed of 4,000 m/min or more. In this method, the fiber is wound around the heating roller by less than one turn in order to ensure threadline stability on the roller but this impairs the uniformity of heat treatment and causes unevenness in various aspects of the yarn such as dyeability. Japanese Patent Application (OPI) No. 143728/78 describes a method wherein undrawn filaments having a crystallinity (Xc) of 30% or more are drawn in the absence of heat at low draw ratios between 1.05 and 1.35. However, crystallization has progressed to a certain extent in the fiber before it is drawn, so that unevenness of yarn may occur if it is subsequently drawn in the absence of heat. Cold drawing has the additional disadvantage that it gives rise to a drawn fiber that is unsatisfactory in both orientation and crystallinity.
The structure of the amorphous portion of a fiber, in particular its orientation, has been reviewed, for example, in Japanese Patent Application (OPI) No. 52721/78 which describes a polyester fiber suitable for processing into woven or knitted fabrics. However, this fiber is extremely low in the density and birefringence and hence is unsatisfactory in strength and elongation. Similar physical properties are specified in Japanese Patent Application (OPI) No. 147814/78; the fiber described in this patent has relaxed orientation in the amorphous portion but is still unsatisfactory in terms of strength (&lt;4.0 g/d) and elongation (.gtoreq.40%). A description of the physical properties of the amorphous portion is also found in U.S. Pat. No. 4,156,071, but the fiber described in this patent is low in the degree of amorphous structure formation and crystallinity (low density) and hence has low-strength, high-elongation, and low-modulus characteristics. Japanese Patent Application (OPI) No. 121613/82 also includes a description regarding the structure of the amorphous portion, but the fiber proposed has an extremely high degree of crystallinity (Xc) according to an X-ray method and an excessively low shrinking stress, so that the heat settability of the fiber is too low to ensure high efficiency and good results in subsequent processing such as crimping.
As described above, various proposals have been made in order to enable a single step of high-speed spinning to produce a yarn whose quality is comparable to that of drawn fibers. However, the fibers produced by the thus far described methods are defective in one way or another as manifested by insufficient strength and elongation properties, reduced dyeability or high likelihood of unevenness of occurring in yarn on account of thermal shrinking stresses.
It is well known that the progress of crystallization during high-speed spinning is usually dependent on the rapidity of spinning operations and a sudden increase in the crystallization rate in the neighborhood of 4,000 m/min has been reported as described, for example, in Sen-i Kikai Gakkaishi (Journal of the Textile Machinery Society of Japan), Vol. 38, p. 268 (1985). As for the effects of air drag on the progress of crystallization, the relationship between the tension during spinning and the distance of fiber travel (i.e., the distance between the spinneret and the convergence point as defined in accordance with the present invention) and part of the relevant physical data have been reported in the proceedings of the 10th Joint Conference of Textile Societies in Japan (Oct. 11-12, 1984) on pages 84 and 85. According to this reference, the fiber forms an amorphous structure as it travels an increased distance, but the reference does not have any description of a heat treatment to be conducted in subsequent stage and it remains entirely unknown what changes will occur in the fiber structure or its strength and elongation characteristics as a result of heat treatment. As is shown later in this specification, ease of handling during spinning is not attainable if the spinning speed becomes higher than 6,000 m/min, and at a speed of 4,000 m/min the strength and elongation properties of the fiber taken up show so much deterioration that no significant improvement will be achieved even if the fiber is subjected to subsequent heat treatment.